Research Disclosure, Volume 104 , published December 1972, Item 10442, titled "Selective Reversal Sensitization of Spectrally Sensitized Silver Chloride", discloses a photographic element capable of forming either a positive or negative image, depending upon the wavelengths and sequence of exposures. The exposures are believed to control the surface pAg of a photographic silver chloride emulsion so as to supersensitize direct-positive image formation in the spectrally sensitized region of the spectrum or enhance the negative image-forming capability in the region of absorption of the silver chloride. In employing this approach to forming a positive or negative image no desensitizer is disclosed in association with the silver chloride emulsion.
In Gilman et al U.S. Pat. No. 3,772,030, issued Nov. 13, 1973, a photographic emulsion and element are disclosed capable of producing direct-positive photographic images with an internally fogged photosensitive silver halide having grains free of surface fog and having on the surface of the silver halide grains a desensitizing compound having a polarographic reduction potential less negative than -1.00 volt and a polarographic oxidation potential more positive than +0.76 volt. Desensitizing compounds are thought to be useful in concentrations of from 10 to 800 milligrams per mole of silver. In one optional form a sensitizing dye can be employed in combination with the desensitizer. In the only specific teachings of emulsions containing both desensitizing compounds and sensitizing dyes, the desensitizer is present in a concentration of at least 200 milligrams per mole of silver in an emulsion having an average grain size of 0.35 micron in diameter. This corresponds to an average surface coverage of approximately 50 percent of the surface area of the silver halide grains.